This invention relates to the infrared heating of preforms having a fairly thick wall made of plastic, especially polyethylene terephthalate (PET), for the blow moulding or stretch-blow moulding of receptacles like bottles, flasks or the like, and especially to improvements in the methods and apparatuses that implement such heating.
In the prior art, the preforms are heated in scanning furnaces fitted with infrared radiation lamps arranged along the trajectory followed by the preforms, which can also be rotated about their axes, in order to be heated in a uniform fashion. The preforms are heated solely from the outside and, in view of the fairly significant thickness of their walls, the result is the formation of a temperature gradient of sizable value between the external and internal sides of the walls, the internal sides being cooler than the external sides.
During the blow moulding or stretch-blow moulding process which, based on the heated preform, leads to a receptacle with a thin wall, the rate of stretching is different for the internal side and for the external side of the preform wall, the internal side having the highest stretching rate. For example, with a cylindrical receptacle 80 mm in diameter blown from a preform with an internal diameter of 12 mm and an external diameter of 20 mm, the crosswise stretching rate (equal to the ratio of the final diameter over the initial diameter) varies from 6.6 for the internal side of the cylinder to 4 or its external side. However, even though the internal side is the most distorted, it is the one that is the coolest in the heating processes in use. The result is the possibility of faults occurring (ungluing of the internal and external layers of the wall), which are more likely the greater the thickness of the wall.
This problem becomes more serious with reusable bottles, flasks, etc. because the physical/chemical constraints produced by the cleaning operations between two successive uses require fairly substantial thicknesses (for instance 0.8 mm) for the walls of the receptacles, which equates to a thickness of about 4 to 8 mm for the walls of the preforms.
One solution might be altering the heating conditions so that the internal side of the preforms reaches a satisfactory temperature. However, this leads to another major problem whereby the external side reaches an excessively high temperature whereat an undesirable modification of the physical-chemical features of the plastic material occurs. Attempts have been made to attenuate this difficulty by arranging the heating means so that they could approximately homogenize the temperature along the entire thickness of the wall, but this does not resolve the problems associated with the different stretching rates on the internal and external sides of the wall of the preform, or prevent the chance of incidents which might stem therefrom.
Furthermore, an increase in the temperatures translates into an increase in the consumption of electrical energy and/or an increase in the length of exposure of the preforms, and therefore results in a slowdown of the rate of production. At the very least, the result is an increase in the manufacturing costs which is inadmissible.